Message paths: hold the fan's shape still across the selection

The dialog ranked layers by selection (priority 3 for the picked path, 2 for the
rest), so every ↑↓ re-elected the spine and re-laid the whole graph out. Rank by
first-heard order instead and move `emphasis`, the way the node detail's Routes
tab already does — the geometry is fixed by evidence, only the white lane moves.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

meshterm/ui/message_paths_screen.py

@@ -278,15 +278,22 @@ class MessagePathsScreen(Screen):
         (:func:`~meshterm.ui.widgets.route_graph_style`): endpoints named, relays their
         own map marker where the type is known, each labelled by its first hash byte in
         the node's own hue.
+
+        Layout rank is fixed by first-heard order (the first path heard is always the spine),
+        exactly as the node detail's Routes tab fixes it by evidence order, so the fan's
+        geometry holds still as ↑↓ walk the arrivals — only the emphasis (which lane draws
+        white and on top) follows the pick.
         """
         selected = self._arrivals[self._index].hops
+        paths = self._paths()
         layers = [
             PathLayer(
                 hops=path,
                 color=_EDGE_SELECTED if path == selected else _EDGE_UNUSED,
-                priority=3 if path == selected else 2,
+                priority=len(paths) - i,
+                emphasis=1 if path == selected else 0,
             )
-            for path in self._paths()
+            for i, path in enumerate(paths)
         ]
         glyph_of, label_of, label_rgb_of = route_graph_style(
             resolve=self._resolve, self_name=self._self_name, source=self._source,

tests/test_message_paths_screen.py

@@ -96,6 +96,27 @@ def test_paths_screen_draws_selected_path_white_over_gray() -> None:
     assert "38;2;255;255;255" in raw2 and "38;2;110;110;110" in raw2
 
 
+def test_paths_screen_graph_geometry_holds_still_across_the_selection() -> None:
+    """↑↓ repaint the fan's colours, never its shape — the Routes tab's rule.
+
+    Layout rank is first-heard order, so only ``emphasis`` follows the pick; the drawn
+    glyphs/lanes are byte-identical from every selection.
+    """
+    now = utcnow()
+    arrivals = [
+        Arrival(when=now, hops=("3d63",), snr=4.0),
+        Arrival(when=now + timedelta(seconds=2), hops=("a1b2", "77aa"), snr=-2.0),
+        Arrival(when=now + timedelta(seconds=4), hops=("a1b2", "5c5c", "77aa"), snr=1.0),
+    ]
+    screen = _screen(arrivals)
+    shapes = []
+    for _ in range(len(arrivals)):
+        shapes.append(_plain(screen._graph_lines(76)))
+        screen.handle("down")
+    assert len(set(shapes)) == 1, "the graph's shape moved when the selection did"
+    assert shapes[0].count("\n") > 1  # a genuine multi-lane fan, not one flat line
+
+
 def test_paths_screen_marks_a_repeater_relay_with_its_triangle() -> None:
     """A relay whose type resolves to a repeater draws ▲, not the generic dot."""
     screen = _screen(_arrivals(), type_of=lambda h: 2 if h == "3d63" else None)